Acetylene can be used as a chemical precursor or as a feedstock for industrial combustion uses, such as welding and metal cutting. Commercial production of acetylene has been carried out since the early twentieth century. The original method for acetylene production utilized coal as the source material, through a process involving a calcium carbide intermediary. Other methods were developed later in the twentieth century, mainly using heat-based processes such as thermal cracking or electric arc furnaces.
Acetylene produced from coal involves a three-step process: first, coal is heated to produce high-carbon-content coke; second, the coke is heated further in the presence of calcium oxide to yield calcium carbide; third, calcium carbide reacts with water to yield acetylene and calcium hydroxide. The first two steps require very high temperatures, while the last step is exothermic. This method for forming acetylene is still used commercially, especially in China where coal is readily available.
This process, however, carries the impurities of the coal and lime source materials into the final product, so that the resulting acetylene is contaminated with impurities such as phosphines, arsines, and hydrogen sulfate. All of these species are capable of poisoning catalysts for subsequent chemical reactions, so that they need to be scrubbed from the acetylene product before it can be used commercially. Chemical grade acetylene, used for further chemical processing, must be >99.6% pure C2H2, with <25 ppm phosphine/arsine/H2S. Industrial grade acetylene, which is burned for welding and metal cutting applications, can tolerate more impurities (>98.0 pure C2H2, <500 ppm phosphine/arsine/H2S). Therefore, the coal-derived production of acetylene is limited in the U.S. to forming industrial grade acetylene; still, even when coal-derived acetylene is just used for welding and metal cutting, the presence of potentially hazardous contaminants raises concerns.
As an alternative, acetylene can be prepared from hydrocarbons by partial oxidation, for example by the process developed by BASF, as described in U.S. Pat. No. 5,824,834. In this process, a hydrocarbon feedstock and oxygen are preheated and then reacted in a combustion chamber, causing the produced gases to reach temperatures>1500° C. The combustion reaction is quenched with water to effect rapid cooling, yielding a gaseous mixture (called “cleavage gas”) of acetylene, hydrogen, carbon monoxide, steam and byproducts. This method of acetylene production yields about 7.5% acetylene, along with large quantities of hydrogen (57%), carbon monoxide (26%), and methane (5.2%). One of the byproducts is soot, which needs to be removed from the cleavage gas as it is processed further. Other byproducts include higher-order hydrocarbons, including alkanes, alkenes, alkynes, and aromatics. Removing the impurities from the cleavage gas and recovering the acetylene it contains involve significant engineering challenges.
In addition to the production issues, acetylene is difficult to handle and transport. It is highly explosive. When transported through pipelines, it is kept at a low pressure and is only conveyed for short distances. Acetylene for industrial purposes is pumped into tanks at high pressure and dissolved in solvents, for example, dimethylformamide, N-methyl-2-pyrrolidone, or acetone. When the acetylene cylinder is opened, the dissolved gas vaporizes and flows through a connecting hose to the welding or cutting torch. The entire amount of acetylene in a cylinder is not usable however, because a certain amount remains dissolved in the solvent and is returned to the manufacturer in this state. With the rise of the petrochemical industry in the mid-twentieth century, acetylene continued to be used industrially (i.e., for welding, metal cutting and the like) but it was displaced as a precursor for chemical reactions, replaced by other feedstocks (e.g., ethylene) that were derived directly from oil rather than from coal. As oil has become more expensive and natural gas has become cheaper though, there is increased interest in acetylene as a platform for further chemical processing instead of petroleum-derived feedstocks.
Moreover, the abundance of natural gas is driving the search for more ways to use this material without burning it, to decrease its greenhouse gas effects and to avoid transforming it into CO2, another greenhouse gas, by simple combustion. Increasing demand for non-hydrocarbon sources of fuel supports the use of natural gas as a feedstock for producing hydrogen, which in turn can be used as a source of power. Conventional technologies already exist for extracting hydrogen gas from the methane in natural gas. Steam reforming, for example, can produce hydrogen gas and carbon monoxide; the hydrogen created by the steam reforming process can then be used in pure form for other applications, such as hydrogen fuel cells or gas turbines, in which it combines with oxygen to form water, without greenhouse gas emissions. Other processes, such as partial oxidation, can produce a hydrogen-containing syngas, a combustible mixture that can be used as a fuel. Conventional techniques for producing hydrogen from methane have drawbacks, however. Steam reforming is carried out at high temperatures, and is energy-intensive, requiring costly materials that can withstand the harsh reaction conditions. Steam reforming uses catalysts to effect the conversion of methane to hydrogen, but the catalysts are vulnerable to poisoning by common contaminants. Partial oxidation is a less efficient technique than steam reforming for producing hydrogen, being prone to soot formation, and being limited in hydrogen yield.
Besides natural gas, other mixed gas sources such as oceanic clathrates, coal mine gas, and biogas contain methane gas as well. Biogas is naturally produced mixed gas source that is produced by the anaerobic decomposition of organic waste material in various human-created environments such as landfills, manure holding ponds, waste facilities, and the like, and in natural environments such as peat bogs, melting permafrost, and the like. The anaerobic bacteria that occur in such environments digest the organic material that accumulates there to produce a gas mixture composed mainly of carbon dioxide and methane. Biogas with a high methane content, as can be found in landfill-derived gas mixtures, can be hazardous, because methane is potentially flammable. Moreover, methane is a potent greenhouse gas. Currently biogas that is collected from organic decomposition (e.g., landfills, waste facilities, holding ponds, and the like, or natural regions containing decaying organic materials) can be purified to remove the CO2 and other trace gases, resulting in a high concentration of methane for producing energy. However, simply burning methane-rich biogas produces CO2, another greenhouse gas. It would be desirable to identify uses for biogas or other mixed gas sources that can exploit their energy potential without burning them, to decrease the greenhouse gas effects of methane while avoiding transforming methane into another greenhouse gas, CO2.
There is a need in the art, therefore, for a process that utilizes mixed gas sources such as natural gas or biogas, and/or more purified hydrocarbon feedstocks (e.g., methane, ethane, propane, and butane, and combinations thereof) to form higher-value products. For those processes intended to produce acetylene, it would be advantageous to use mixed gas sources such as natural gas or biogas, and/or more purified hydrocarbon feedstocks (e.g., methane, ethane, propane, and butane) as a feedstock, avoiding the limitations of other mixed gas conversion processes or hydrocarbon combustion processes while taking advantage of the abundance of these feedstock materials. Concomitantly, there is a need in the art for a process that can produce acetylene in a convenient and cost-effective way, using mixed gas sources such as natural gas or biogas, and/or more purified hydrocarbon feedstocks. It would be especially advantageous to produce acetylene with minimal impurities, so that it can be used safely and without substantial additional processing. Furthermore, there is further a need in the art to provide alternative fuels such as hydrogen scalably and efficiently. It would be desirable to carry out these processes in an economic and environmentally responsible way.
In addition, acetylene has utility as a fuel for various industrial applications, for example, metal cutting. This use represents a significant market, comparable in size to various petrochemical uses of acetylene. At present, a major industrial use of acetylene is as a fuel for oxyacetylene torches, used for cutting steel; in addition to cutting, acetylene is used in some welding, carburization, and heat-treating of steel. Oxyacetylene torches burn at a higher flame temperature (3,500° C.) than other oxy-fuel torches, such as oxy-hydrogen (3,000° C.) and oxy-propane (2,500° C.) torches, and oxyacetylene forms a smaller, more precise flame cone. These features allow for higher quality and more precise cutting than other comparable oxy-fuel cutting methods. Additionally, because the combustion of acetylene requires a smaller stoichiometric ratio of oxygen than other fuels like propane, the oxy-acetylene torches consume less oxygen than other oxy-fuel torches, leading to lower oxygen operational costs. Finally, the lower flame temperature and higher oxygen requirements of other hydrocarbon fuel types like oxy-propane torches allow for a higher risk of incomplete combustion, producing hazardous carbon monoxide in the work environment. For the aforesaid reasons, oxy-acetylene cutting is standard in the industry for steel cutting.
However, as described previously, there are limitations in the production of acetylene and its transportation. Therefore, sourcing acetylene for industrial cutting is expensive and logistically challenging. First of all, acetylene used as a fuel for torches must be transported and stored in small metal cylinders because of the risk of explosion. In order to reduce the risk of explosion, the acetylene in the cylinders is dissolved in acetone, lowering its partial pressure and thus the likelihood of explosion. Because acetone is present in the cylinders along with acetylene, the acetylene can only be drawn at low flow rates (for example, not to exceed 1/7 of the container contents per hour), to reduce the chance of acetone being drawn into the outflow line along with the acetylene—acetone in the gas feed can diminish flame temperatures and the quality of the cutting process. Even with low rates of outflow, the acetylene in the cylinders can be depleted quickly; once depleted, a cylinder cannot be refilled on-site without extensive safety infrastructure and expertise, again because of the risk of explosion. Because of their small size, cylinders do not scale well for larger operations, but instead must be connected in parallel via manifolding, adding to a project's complexity. Also, because of the risk of explosion, cylinders require a number of safety precautions as they are transported, adding costs and logistical challenges.
There remains a need in the art for a more streamlined, safe method of sourcing acetylene. It would be desirable to circumvent the need for acetone-containing cylinders as the repository for acetylene gas that is used in metal working. For example, it would be useful to have acetylene fuel available on demand and as needed, avoiding the volume and flow rate constraints of cylinder storage. In addition, it would also be advantageous to have acetylene produced in proximity to the point of its use to avoid the cylinder-specific difficulties with transportation.